The present invention relates to an expandable osteosynthesis cage.
The technical field of the invention is that of implantable bone implants or prostheses and the surgical techniques for using them.
The main application of the invention is to provide implants designed to be slid or inserted from a posterior direction between the facing faces of two consecutive vertebrae in order to maintain a given distance between them and to restore the stability of the spinal column, e.g. after a failure of the corresponding joint, by fixing the two vertebrae together.
Several techniques are known at present for restoring a xe2x80x9cnormalxe2x80x9d lumbar lordosis in this way, by implanting either a graft which in time fuses the vertebrae together, or a prosthesis which fixes them together immediately, while still also making it possible in time to achieve fusion between the vertebra.
In the second above-mentioned technique, use is made mainly of implants, also known as xe2x80x9ccagesxe2x80x9d, some of which are hollow, rigid, and one-piece, with inside/outside intercommunication slots for receiving a bone graft which, via said slots, subsequently fuses with the adjacent vertebrae on either side: in this field, reference can be made to patent application WO 96/08205 published on Mar. 21, 1996 for a xe2x80x9cIntervertebral fusion cage of conical shapexe2x80x9d and application EP 637 440 published on Feb. 8, 1995 for an xe2x80x9cIntersomatic implant for the spinal columnxe2x80x9d. Nevertheless, cages of those types are of outside dimensions that are given and fixed, whereas the distances between pairs of vertebrae are not constant. In addition, the inclinations of the facing vertebral faces to which a given angular position is to be imparted do not enable rigid cages to be used from a posterior direction: they can be inserted only from an anterior direction.
As a result, other types of cage have been developed with two substantially parallel branches connected to a rigid body through which it is possible to turn a wormscrew system which then moves a wedge in screw engagement on said screw from an initial position close to the distal ends of the branches towards the body linking the branches together, thereby splaying the two branches apart angularly. It is then possible to insert such a cage of initially flat shape between the vertebrae, and then by turning the drive axis of the wedge, the desired angle between the branches is adjusted or set from a posterior access.
Such cages or implants are described, for example, in European patent application EP 664 994 published on Aug. 2, 1995, entitled xe2x80x9cVertebral intersomatic cagexe2x80x9d or in application EP 2 719 763 published on Nov. 17, 1995, and entitled xe2x80x9cVertebral implantxe2x80x9d.
Nevertheless, such devices which are more mechanical than hollow and rigid cages, and therefore more complex, leave a smaller inside volume for the fusion graft, and because of their flat shape which is not circularly symmetrical, even though they are better at ensuring a given bearing angle between the vertebrae, they require a passage of the same rectangular section to be prepared to receive them, and that complicates implementation.
The problem posed is thus to be able to have implants or cages available making it possible simultaneously to ally the shape of a conventional rigid cage, firstly to facilitate implantation and secondly to provide a larger inside volume, with the possibility of increasing the diameter of the distal end of the cage to a given value relative to its end situated adjacent to its point of surgical insertion, after it has been put into place, and corresponding to the posterior face of the vertebrae, while having as few mechanical elements as possible.
A solution to the problem posed is an expandable osteosynthesis implant having branches each connected at one end to a seat pierced by an orifice, such that said branches and the seat constitute a hollow cage which, in a xe2x80x9crestxe2x80x9d position, has an outside general shape that is cylindrical or quasi-cylindrical with the generator line that generates it by rotating about its axis of symmetry being either a straight line or having a curved middle portion such as a convex circular arc of large radius, and having a director curve around which the generator line travels, thus also defining the cross-section of the cylinder, which is quasi-circular: this provides a cylinder that is either a right circular cylinder or else a cylinder that is referred to in the present case as being xe2x80x9covoidxe2x80x9d or xe2x80x9covalxe2x80x9d, and which is referred to below as being a cylinder or a quasi-cylinder; a portion at least of the inside volume of the cage towards the distal ends of the branches is in the form of a quasi-circular truncated cone whose large base is towards said seat, said implant having at least three branches and at least one spacer of dimensions compatible with the dimensions of the large base of the truncated cone in said inside volume in the rest position, and possibly suitable for passing through said orifice.
In an xe2x80x9cactivexe2x80x9d position, said spacer splays said branches apart, said inside volume tends towards a circular cylinder, and the outside shape of the cage tends towards an approximate truncated cone, and once the spacer has been put into the desired position, no internal part remains inside the space defined by said branches, the spacer, and the orifice.
The definitions of the xe2x80x9crestxe2x80x9d and xe2x80x9cactivexe2x80x9d positions are shown by way of example respectively in FIGS. 1, 3A, 4A, 5A, 9, 10, and 11, and in FIGS. 2, 3B, 3C, 4B, and 5B. The xe2x80x9crestxe2x80x9d position is the position of the implant before it is put into place and while it is being put into place, i.e. without the spacer positioned between its branches inside the cage, so the cage has a cylindrical outside volume of constant section. The xe2x80x9cactivexe2x80x9d position is the final position of the implant, e.g. between two vertebrae, with its branches splayed apart by the spacer being placed in its final adjustment position, the general outside shape of the cage then being approximately frustoconical, flaring away from the seat, which corresponds to the end of the implant situated towards its point of surgical insertion, towards its distal end which is placed in deeper between the vertebrae.
To provide better anchoring in the bone and to avoid any subsequent migration of the cage, the outside surfaces of said branches are preferably either knurled, grooved, or threaded using a thread profile having projecting ridges, etc.
Also, to reduce any risk of rotation after implantation, and thus reduce any risk of displacement of the cage, while simultaneously increasing contact area with the faces of the vertebrae, at least the seat of the implant and preferably also the sides of the branches have at least two optionally parallel flats, each disposed symmetrically about the axis of the implants between two successive branches. These flats, optionally assisted by the generally ovoid shape of the cage, make better retention possible after expansion by reducing any risk of the implant rotating. In addition, said ovoid shape can make it possible, better than if the outer generator line of the basic cylindrical shape of the cage at rest were a straight line, to return after expansion to an outside profile that is conical and without curvature, thereby providing better-distributed thrust against the body of the vertebrae, thus helping the bone graft to take better.
To stiffen the cage at its posterior end, particularly if there is a large orifice in the seat, thereby making it easier to fill the cage with bone matter after the cage has been put into place and expanded, the orifice of said seat is suitable for receiving a plug for closing the inside volume of the cage. By way of example, the plug can be screwed into said orifice, in which case the orifice is also threaded. Under such circumstances, the plug prevents bone matter from escaping and depending on the material out of which it is made, it can also stiffen the cage.
Various particular embodiments are described below in the accompanying figures. The result is novel expandable osteosynthesis implants satisfying the problem posed, in particular for the above-defined main application.
The presence of at least four branches, and possibly four to eight branches, makes it possible to obtain bilateral expansion, and thereby better jamming against the two facing faces of the vertebrae, and the absence of a link part or rod between the spacer maintaining said expansion and the orifice or plug in the end seat guarantees a larger inside volume for receiving a large quantity of bone matter, thereby improving consolidation and joining by fusion, in particular between the vertebrae that are to be held together. Said filling operation is made that much easier by having a large orifice at the posterior end of the cage through said seat.
In addition, said orifice makes it possible to scrape the faces of the vertebrae through the slots situated between the branches in the bottom and top faces of the cage.
Furthermore, the expansion system as defined above is very simple, since it comprises only two parts, namely the spacer and the cage (optionally having a plug situated at its end and possibly also having an intermediate spacer, as mentioned below) but no link member remaining after installation, since any rod or shaft for positioning said spacers an aid plug is subsequently removed.
The present invention thus provides numerous advantages over existing implants or cages, of the kind already mentioned, and other advantages can also be provided but those given above suffice to demonstrate the novelty and the usefulness of the invention.